Adds initial Taskbar Edu implementation.

Currently it just pops up with one placeholder image for splitscreen.
The Next button doesn't do anything.

Screenshots:
Light: https://screenshot.googleplex.com/B27CwBcwobHTghN.png
Dark: https://screenshot.googleplex.com/48EuJZv8evd5RGW.png

Known issues:
 - Back gesture is picked up behind by the app behind the taskbar.
 - When the flag is enabled, the Edu shows up each time you open
   an app, rather than only the first time.

Bug: 180605356
Test: Manual
Change-Id: Ifba9aa59abf1501b5f8ddebb3fcc13df85f6efaa